SAW filters for reception applications frequently require an unbalanced input and a balanced output so that they can be used without an additional balun between an unbalanced antenna connection and a transceiver IC with balanced signal processing. Such a reception filter can also be interconnected with a further filter operating in an unbalanced/unbalanced manner to form a duplexer. However, it is also possible to connect up a filter operating in a balanced/balanced manner to a balun, so that this connection to input and output can then likewise be operated in a balanced/unbalanced manner.
In a duplexer, further demands are made on the reception filter, for example a high level of common-mode isolation of greater than 60 dB between the transmission and reception filters and a high level of normal-mode isolation of more than 60 dB in the transmission band are demanded. Furthermore, a general aim in the case of filters is for them to be implemented as inexpensively as possible and to be equipped with as little insertion loss as possible. Furthermore, a high level of balancing is demanded at the balanced output, and variances are also intended to be at a minimum.
The U.S. Pat. No. 7,369,015 B2 discloses a SAW filter operating in a balanced/unbalanced manner which comprises two series-connected DMS tracks. Further known reception filters use a series circuit of a ladder type filter and a single-track DMS filter. Disadvantages of these interconnections are an inadequate level of isolation and a large chip surface area requirement.